1. Field of the Invention
The invention relates to an image forming apparatus such as a copying machine or a printer for forming an image on a recording material by the utilization of an electrophotographic process.
2. Related Art
In recent years, as a plural-color or full-color image forming apparatus of the electrophotographic type, there has been put into practical use a so-called in-line type image forming apparatus in which a plurality of photosensitive drums are arranged in a line in conformity with respective colors, and toner images of the respective colors formed on the respective photosensitive drums are successively superimposed on an intermediate transfer belt to thereby form a color image.
As an example of the conventional art, in Japanese Patent Application Laid-Open No. 2001-117383, there is described a full-color image forming apparatus (full-color printer) 101 of a conventional electrophotographic type having an in-line type intermediate transfer belt (intermediate transferring means) 120 as shown in FIG. 6 of the accompanying drawings.
This image forming apparatus 101 is provided with four image forming portions (image forming units), i.e., an image forming portion 110Y for forming a yellow image, an image forming portion 110M for forming a magenta image, an image forming portion 110C for forming a cyan image, and an image forming portion 110Bk for forming a black image, and these four image forming portions 110Y, 110M, 110C and 110Bk are arranged in a line at predetermined intervals.
In the respective image forming portions 110Y, 110M, 110C 110Bk, there are installed drum-shaped electrophotographic photosensitive members (hereinafter referred to as the photosensitive drums) 111(111Y, 111M, 111C and 111Bk) as image bearing members. Around the respective photosensitive drums, there are disposed primary chargers 112(112Y, 112M, 112C and 112Bk), developing devices 113(113Y, 113M, 113C and 113Bk), transfer rollers 115(115Y, 115M, 115C and 115Bk) as transferring means, and drum cleaner devices 114(114Y, 114M, 114C and 114Bk), and a laser exposure device 140 is installed below the primary chargers 112 and the developing devices 113.
In the respective developing devices 113, there are contained a yellow toner, a magenta toner, a cyan toner and a black toner.
Each photosensitive drum 111 is a negatively charged OPC photosensitive member and has a photoconductive layer on a drum base made of aluminum, and is rotatively driven at a predetermined process speed in a counter-clockwise direction by a driver (not shown).
The primary chargers 112 as primary charging means uniformly charge the surfaces of the respective photosensitive drums 111 to a predetermined potential of negative polarity by a charging bias applied from a charging bias voltage source (not shown).
Each developing device 113 contains the toner therein and causes the toner of each color to adhere to an electrostatic latent image formed on each photosensitive drum 111 to thereby develop (visualize) the latent image as a toner image.
The transfer rollers 115 as primary transferring means are disposed in respective transferring portions so as to be capable of contacting with the respective photosensitive drums 111 with the intermediate transfer belt 120 interposed therebetween.
The drum cleaner devices 114 have cleaning blades or the like for removing any untransferred residual toner on the respective photosensitive drums 111 during primary transfer from the photosensitive drums 111.
The intermediate transfer belt 120 is passed over two belt conveying rollers 121 and 122, and is driven in a clockwise direction in FIG. 6. This intermediate transfer belt 120 is formed of dielectric resin such as polycarbonate, polyethylene terephthalate resin film or polyvinylidene fluoride resin film.
The secondary transfer opposed roller (conveying roller) 121 is disposed in a secondary transferring portion so as to be capable of contacting with a secondary transfer roller 130 with the intermediate transfer belt 120 interposed therebetween. Also, outside the endless intermediate transfer belt 120 and near the conveying roller 122, there is installed a belt cleaning device 123 for removing and collecting any untransferred toners remaining on the surface of the intermediate transfer belt 120. Also, a fixing device 103 having a fixing roller 103a and a pressure roller 103b is installed downstream of and above the secondary transferring portion with respect to the transport direction of a transfer material P and forms a substantially vertical transport path.
The exposure device 140 is comprised of laser beam emitting means for effecting beam emission corresponding to the time-serial electrical digital pixel signal of given image information, a polygon lens, a reflecting mirror, and the like, and exposes each photosensitive drum 111 to the laser beam to thereby form an electrostatic latent image for each color conforming to the image information on the surface of each photosensitive drum 111 charged by each primary charger 112.
By the above-described construction, the toner images formed on the surfaces of the respective photosensitive drums 111 are successively primary-transferred onto the intermediate transfer belt 120, whereby a color image is formed thereon, and this color image is secondary-transferred to the transfer material P transported from a cassette 102, by the secondary transfer roller 130 whereafter it is fixed on the transfer material P by the fixing device 103, and the transfer material P is delivered onto a delivery tray 100a outside the image forming apparatus 101 by delivery rollers 128.
As described above, in the construction as shown in FIG. 6 wherein the fixing device 103 is disposed above the secondary transferring portion and the transfer material P is delivered onto the delivery tray 100a on the upper surface of the main body of the image forming apparatus 101 with the recording surface thereof facing down, the transport path is short and an improvement in throughput can be achieved and the transport path concentrates in one location, and this leads to the feature that jam clearance is easy to do.
In the above-described conventional image forming apparatus 101, however, the total height of the apparatus main body is determined by the height of the feed cassette 102, the exposure device 140, the photosensitive drums 111 and the intermediate transfer belt 120 as they are stacked one upon another, the distance from the secondary transferring portion to the fixing device 103, and the height from the fixing device 103 to the delivery rollers 128 as a delivery portion, and it has been difficult to make the height of the apparatus main body small.
Particularly, in the transport path of the transfer material P between the secondary transferring portion and the fixing device 103, in the secondary transferring portion, in order to accurately transfer the toner image on the intermediate transfer belt 120 to the transfer material P, and in the fixing device 103, in order to fix the toner image on the transfer material P by heat and pressure, in order that in both of them, the nipping force may be strong and the transfer material P may be transported without slipping, it is sometimes practiced to set the transport speed in the fixing device 103 so as not to be low relative to the transport speed in the secondary transferring portion, and transport the transfer material P while being slowed down between the secondary transferring portion and the fixing device 103. For this purpose, the distance between the secondary transferring portion and the fixing device 103 is determined in conformity with the length and stiffness of the transfer material P and therefore, it has been difficult to simply lower the position of the fixing device 103.
Also, in recent years, the compactness, space saving and lower cost of the product are required, and it has been one of the most important items to make the size of the apparatus main body small.
In Japanese Patent Application Laid-Open No. 2002-144633, there is described a construction in which an intermediate transfer belt passed over two rollers is inclined with its secondary transferring position facing down. However, this is a construction in which a primary transferring surface is disposed upwardly and as a result, the distance from the secondary transferring position to a transfer material delivery portion becomes long, and a reduction in throughput is caused and jam clearance becomes difficult.